These pill are making me not eat help?

i'm 13 years old and i started taking these pills called Ritalin and welbutrin.. when i statred taking them i was 117 pounds.. (a little over healthy but i'm a skinny girl) then went down to 111 now i'm at 100.. and i cant eat while taking theses pills.. idk why they just make me not have a apatite .. and im scared. i'm always shakey and have head aches